Various electro-optical systems have been developed for reading optical indicia, such as bar codes. A bar code is a coded pattern of graphical indicia comprised of a series of bars and spaces having differing light reflecting characteristics. The pattern of the bars and spaces encode information. In certain bar codes, there is a single row of bars and spaces, typically of varying widths. Such bar codes are referred to as one dimensional (1D) bar codes. Other bar codes include multiple rows of bars and spaces, each row typically having the same width. Such bar codes are referred to as two dimensional (2D) bar codes and examples include PDF417 and DataMatrix bar code formats. By virtue of the use of multiple rows of features (bars and spaces), 2D bar codes typically allow more encoded information than a 1D bar code in a given bar code area. Devices that read and decode one and two dimensional bar codes utilizing imaging systems that image and decode imaged bar codes are typically referred to as imaging-based bar code readers or bar code scanners.
Imaging systems include charge coupled device (CCD) arrays, complementary metal oxide semiconductor (CMOS) arrays, or other imaging pixel arrays having a plurality of photosensitive elements or pixels. An illumination system comprising light emitting diodes (LEDs) or other light source directs illumination toward a target object, e.g., a target bar code. Light reflected from the target bar code is focused through a lens of the imaging system onto the pixel array. Thus, an image of a field of view of the focusing lens is focused on the pixel array. Periodically, the pixels of the array are sequentially read out generating an analog signal representative of a captured image frame. The analog signal is amplified by a gain factor and the amplified analog signal is digitized by an analog-to-digital converter. Decoding circuitry of the imaging system processes the digitized signals and decodes the imaged bar code.
In an image based scanner it is helpful to the decoding performance to minimize blur in the image. Because the scanner of the item being scanned or both are potentially in motion, a minimum exposure time is used to minimize blur. On a CMOS sensor, lowering exposure time for high pixel count imagers increases the gain used on the sensor electronics when sampling pixels. This high gain introduces noise and aberrations noticeable to the eye in the image. This is acceptable for a barcode decoder, but degrades the image when using the device as a camera.